In the field of medical diagnostics and research, the detection, identification, quantification, and characterization of cells of interest, such as cancer cells, through testing of biological samples is an important aspect of diagnosis and research. Typically, a biological sample such as bone marrow, lymph nodes, peripheral blood, cerebrospinal fluid, urine, effusions, fine needle aspirates, peripheral blood scrapings or other biological materials are prepared by staining a sample to identify cells of interest.
In Fluorescent In Situ Hybridization (FISH) a fluorescently labeled oligonucleotide probe is added to a tissue sample on a microscope slide under conditions that allow for the probe to enter the cell and enter the nucleus. If the labeled sequence is complementary to a sequence in a cell on the slide a fluorescent spot will be seen in the nucleus when the cell is visualized on a fluorescent microscope. FISH has the advantage that the individual cells containing the DNA sequences being tested can be visualized in the context of the tissue.
Immunostaining techniques utilizing non-fluorescent techniques are also commonly used. Such techniques can include the formation of colored precipitates and enzyme based reaction to label a sample. The result of the staining provides, for example, a precipitate at a location comprising a particular biological molecule, cell, or characteristic of interest.
Both non-fluorescent and fluorescent manual staining techniques are time consuming, result in variability among samples, and often utilize hazardous reagents. To overcome these problems automated systems have been designed to introduce cost savings, uniformity of slide preparation, and reduction of errors. Automated slide stainers are widely used in pathology to stain tissue samples that have been cut with a microtome and placed on glass slides. One common type of automated slide stainer consists of a set of racks for holding slides flat over a drain pan and a robotic arm which can travel in x, y and z over the slides. The arm carries a set of fluid dispensers. Some of these are connected to pumps that dispense a single fluid such as buffer. Others are connected to valves and can be used to dispense several fluids and some may have syringe pumps attached so they can draw and dispense fluids from vials prepositioned under the arm's range of motion. Descriptions of exemplary automatic slide stainers can be found in U.S. Pat. Nos. 6,352,861; 6,183,693; 6,349,264; and 6,180,061, the contents of which incorporated herein by reference in their entirety. There are several limiting factors to these designs including (1) that it is helpful to be able to control where the reagent is placed relative to the tissue sample, and (2) some fluids such as wash buffers are poured on the slide in excess and allowed to flow off the sides of the slide into the drain but others such as custom synthesized antibodies for IHC stains are very expensive. Typically just enough of these expensive reagents to cover the tissue sample is dripped onto the slide and held in place by surface tension.